1. Field of the Invention
This invention relates generally to microwave devices, and more particularly, it relates to slow-wave structure assemblies for use in traveling-wave tubes and methods for fabricating such assemblies.
2. Description of the Prior Art including Prior Art Statement
In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic wave energy. In order to achieve the desired interaction, the electromagnetic wave is propagated along a slow-wave structure, such as an electrically conductive helix wound about the path of the electron stream. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure so that the traveling wave may be made to propagate axially at nearly the velocity of the electron stream.
Initially, slow-wave structures of the helix type were usually supported within a tubular housing by means of a plurality of longitudinally disposed dielectric rods equally circumferentially spaced about the slow-wave structure helix. More recently, supporting assemblies for helical slow-wave structures have been devised which employ a coaxial helix of dielectric material wound in the same sense as the slow-wave structure helix between the slow-wave structure helix and the housing.
Helical supporting arrangements for slow-wave structures and methods for fabricating such arrangements are disclosed in U.S. Pat. No. 3,670,196 to Burton H. Smith. As disclosed in the Smith patent, a mandrel is provided with a helical groove conforming to the desired pitch of the slow-wave structure helix, and the slow-wave structure helix is wound in this groove. A dielectric helix provided with suitable braze material is then wound in the aforementioned groove over the slow-wave structure helix, the dielectric helix having a sufficient radial extent to project radially outwardly from the mandrel groove. The assembly is then inserted into a tubular housing, and the circumferentially outer surface of the dielectric helix is brazed to the housing, after which the mandrel is removed by chemical etching.
A modification to the aforedescribed technique for fabricating helical supporting arrangements for slow-wave structures is disclosed in U.S. Pat. No. 4,115,721 to Walter Friz. The process disclosed in the Friz patent is similar to that of the Smith patent, except that instead of mechanically winding a helical dielectric member in the mandrel groove, the dielectric material is plasma sprayed into the groove on the exposed surface of the previously wound slow-wave structure helix, after which the mandrel and the plasma-sprayed dielectric material are precision ground to the desired radial dimension.
A further consideration in the design of traveling-wave tubes is that the interaction between the electron stream and the traveling wave causes a gradual reduction in the axial velocity of the electron stream as it traverses the tube. As a result, the relative axial velocities of the electron stream and the traveling wave may become appreciably different from one another near the output end of the tube, thereby reducing operating efficiency. In the past, for traveling-wave tubes with helical slow-wave structures, this problem has been solved by gradually decreasing the pitch of the helical slow-wave structure along the path of the electron stream to cause the axial velocity of the traveling wave to decrease in a manner corresponding to the decrease in the axial velocity of the electron stream. Helical slow-wave structures of decreasing pitch are disclosed in U.S. Pat. No. 2,851,630 to Charles K. Birdsall.
A further consideration applicable to the design of wide bandwidth traveling-wave tube involves conductively loading the slow-wave structure of the tube to achieve the necessary phase velocity verses frequency characteristic for the traveling waves that facilitates wide bandwidth operation. A representative prior art conductive loading arrangement for increasing the bandwidth of a traveling-wave tube employing a helical slow-wave structure is disclosed in U.S. Pat. No. 3,972,005 to John E. Nevins, Jr. et al. The conductive loading structure of this patent includes a plurality of elongated open-sided conductive channel members extending radially inwardly from the slow-wave structure housing with the open side of each channel member facing the slow-wave structure.